Pathological definition and clinical significance of vascular invasion in thyroid carcinomas of follicular epithelial derivation.

There are many controversies involving the diagnostic criteria and treatment of well-differentiated thyroid carcinoma. Vascular invasion has been identified as an important and independent prognosticator in many cancers. The majority of pathologists recognize the importance of vascular invasion as a diagnostic marker of malignancy in follicular lesions of thyroid; however, several reports have suggested that angioinvasion is not a predictor of bad prognosis in thyroid carcinomas. We suggest that the criteria for diagnosing angioinvasion in thyroid carcinomas as well as in other endocrine tumors are inconsistent and the controversy may be attributed to application of inappropriate criteria. We carried out a study of a potential cause of artefactual vascular invasion in a series of autopsy thyroids and established the morphology of mimics of angioinvasion. We then reviewed retrospectively the clinicopathological features of a series of 4000 thyroid carcinomas of follicular epithelial derivation to identify the features and significance of the most rigid criteria of vascular invasion: tumor cells invading through a vessel wall and thrombus adherent to intravascular tumor. These features were identified in 118 (3%) lesions. Follow-up information was available for 98 patients. Of these, 35% developed distant metastases. When using the rigid criteria, ∼1/3 of angioinvasive well-differentiated thyroid carcinomas and 1/2 of angioinvasive poorly differentiated thyroid carcinomas developed distant metastases at a mean 5.3 years of follow-up. Our results indicate that the application of rigid criteria for vascular invasion provide a clinically relevant prediction of distant metastasis in patients with thyroid carcinomas, especially in well-differentiated thyroid carcinomas.